A major challenge in cell-cycle research is to understand how cell division is regulated in concert with growth and differentiation during the development of complex organisms. Although substantial progress has been made in understanding ligand-induced signaling and cell-cycle regulation individually, it remains unclear how these processes are integrated in vivo to accomplish the proper timing of cell division during animal development. Genetic model systems have provided powerful means to address these questions. In this grant proposal, we describe genetic approaches in the nematode C. elegans aimed at identifying how developmental signals connect to the cell-cycle machinery during the G1 phase of the cell cycle. In preliminary studies, we have characterized G1 cell-cycle genes in C. elegans. These include positive regulators homologous to a D-type cyclin and a cdk4/6 kinase, and negative regulators related to the retinoblastoma (Rb) protein and the p21/p27 CIP/KIP family of CDK inhibitors. Our results indicate that G1 control in C. elegans follows molecular mechanisms that are closely related to those used in mammalian cells. In addition, we have isolated mutations that disturb cell division in specific lineages and developmental stages, likely defining genes that upstream of the cell-cycle machinery. Specific Aim 1 describes genetic screens aimed at identifying genes that act upstream of the basic cell-cycle machinery. Each of the screens has been tested and has already produced interesting candidate mutants. Mutations will be characterized, mapped and assigned to complementation groups. Molecular characterizations, described under specific Aim 2, will initially focus on three previously identified mutations. These mutations allow a subset of cells to divide during the second larval stage and appear to define a pathway for cell lineage dependent regulation of a CIP/KIP CDK inhibitor. These experiments will help reveal cell division-control mechanisms that are poorly understood yet highly important for our understanding of normal development as will as diseases as cancer.